The spectral Doppler technique has been widely applied to noninvasive detection and measurement, especially to the detection and measurement of blood flow in a blood vessel.
In ultrasonic Doppler system, ultrasonic signals are transmitted into a target area of a human body. The transmitted ultrasonic signals are then scattered by the cells of body tissue or those of blood flow in the target area. Part of the scattered signals return back to a receiver of the system, and are converted into electric signals, which are called echo signals. The received echo signals are amplified by an amplifier and analyzed by a series of Doppler processes, to obtain a spectrogram of Doppler signals and valuable indices, such as the velocity of blood flow, for clinic diagnosis.
FIG. 1 is a block diagram of a typical ultrasonic Doppler system. As shown in FIG. 1, the received echo signals are beam-formed and quadrature demodulated to obtain the quadrature Doppler signals. In an ultrasonic Doppler system, the amplitude of the echo signals from the tissue or vascular wall is normally much higher than those from blood flow. For this reason, a high-pass filter (or known as wall filter) is needed to process the obtained quadrature Doppler signals after gap filling. In this way, most of the echo signals from the stationary or near-stationary tissue and vascular wall, which are characterized in high amplitude and extremely low frequency, can be successfully cancelled.
As shown in FIG. 1, after high-pass filtering, the quadrature Doppler signals, in one path, are fed into a spectral analysis unit to calculate the spectrogram. Then, a parameter calculating unit extracts the mean frequency waveform, maximum frequency waveform and etc. based on the spectrogram, thereby producing some valuable indices for clinic use. The spectrogram and the indices, such as the maximum frequency waveform and etc., are then converted by a DSC (Digital Scan Converter) and sent to a monitor for real time display. In the other path, the filtered quadrature Doppler signals are fed into a direction separating unit, where the Doppler signals are separated into forward and reverse components (hereafter referred to as forward and reverse Doppler signals), each of which corresponds to one of the blood flow directions. At last, the separated forward and reverse Doppler signals are converted by a DAC (digital-analog converter) and output to the right and left stereo speakers respectively, so as to output the audio Doppler signals. By using such a Doppler system, doctors can make more accurate diagnosis under the help of the spectrogram displayed in the monitor and the sound from the speaker.
In a spectrogram, the number of frequency points in each spectral line is limited, for example, only 128 points. To obtain the maximum frequency waveform and the mean frequency waveform with high accuracy, the spectrogram is required to fully fill the whole display area. In practical, the velocities of forward and reverse blood flow are usually asymmetrical, and as a result, for example, the bandwidth of forward Doppler signals is double of that for the reverse Doppler signals. In this case, if the spectrogram is still required to fully fill the whole display area, spectral alias will be occurred, that is, the frequency components of the forward Doppler signals will be displayed in the negative frequency range, or vice versa, as shown in FIG. 2. In FIG. 2, the area filled with dots indicates the frequency components of the reverse Doppler signals, and the blank one indicates that of the forward Doppler signals. As shown in FIG. 2, part of the forward Doppler signals are displayed in the negative frequency range of −π˜0, that is, the forward Doppler signals are aliased. FIG. 10a shows the aliasing phenomena in an actual spectrogram. The spectral alias as shown in FIG. 2 will be more obvious in a pulsed wave (PW) Doppler system with low pulse repetition frequency (PRF). In a PW Doppler system, the PRF is usually required to satisfy the following condition:Vrange<(PRF*c)/(2*f0)
Where Vrange is the sum of the maximum velocities of forward and reverse blood flow; c is the sound speed, and f0 is the frequency of the transmitted ultrasound wave. If the velocity of blood flow and PRF meets the above condition, the aliasing phenomena may be cancelled, by adjusting the baseline position of a spectrogram during spectral analysis and causing frequency components of the forward and reverse Doppler signals fully fill the whole display area. The adjustment of baseline position may be implemented directly by moving the frequency spectrum of Doppler signals in frequency domain, or by performing digital frequency modulation on Doppler signals in time domain.
Referring back to FIG. 1, after spectral analysis, the Doppler signals are fed into direction separating unit to obtain the forward and reverse Doppler signals. FIG. 3 shows a typical direction separating unit. As shown in FIG. 3, one quadrature component of Doppler signals, i.e. I(t) is filtered by a Hilbert transform filter, and the other quadrature component Q(t) is delayed by a delayer. Then, the filtered I(t) and delayed Q(t) are added up to obtain the reverse Doppler signals R(t), or performed subtraction to obtain the forward Doppler signals F(t). Since the baseline adjustment in a spectrogram will cause the bandwidth in one frequency direction (positive or negative) greater than PRF/2, the output audio Doppler signals will be distorted if the sampling rate thereof still remains PRF. In order to avoid audio distortion and output dealiased forward blood flow signals, the sampling rate of the output audio Doppler signals has to be increased. For example, the maximum sampling rate of output audio Doppler signals is required to be 2 PRF, for the adjustment of baseline position is up to PRF/2.
U.S. Pat. Nos. 5,553,621 and 5,676,148 disclose some methods for generating dealiased Doppler signals without increasing PRF. FIG. 4 illustrates the disclosed method of U.S. Pat. No. 5,553,621. As shown in FIG. 4, the complex signals I(t)+iQ(t), which consists of two quadrature components of Doppler signals, are up-sampled by being inserted zero between two adjacent samples, so that a complex sequence with doubled sampling rate is obtained. The obtained complex sequence is then digitally modulated so as to allow the band center frequency of forward (or reverse) Doppler signals to be zero frequency. The modulated sequence is then filtered by using a low-pass filter with real coefficients, so as to remove the interference caused by the zero insertion. Next, the filtered sequence is demodulated to restore its frequency characteristic. At last, the operation of Re{.} shown in FIG. 4 will be performed on the demodulated complex sequence to acquire real part of the complex sequence, which is the dealiased forward (or reverse) Doppler signals, or called forward (or reverse) blood flow signals.
However, the above method has the following shortcomings: since the zero insertion is performed on the input signals, the computation amount and the desired memory space during audio signals processing will be increased significantly, which results in high cost and lower practicability. In addition, the above method needs to obtain the analytic signals (single side band signals) of unidirectional (forward or reverse) blood flow signals first, and then output the real part of the analytic signals. This increases the complexity of processing.
Moreover, US2005/0090747 also provides a method for generating dealiased Doppler signals, but without up-sampling processing. That is, the Doppler system determines the aliased frequency points based on the detected maximum frequency, and then automatically performs frequency modulation on the input signals. The drawback of this method is that: the modulated signals may be distorted, since the sampling rate of the output signals are same as that of input ones.